The present invention relates to an autofocus apparatus. More particularly this invention relates to an autofocus apparatus applied in image input equipment such as a digital still camera, a digital video camera or the like that use an image pickup element.
In association with heat-up of competition in the recent digital camera market, cost reduction will become inevitable, thus reduction of a number of components becomes an important issue. Considering the circumstances as described above, there is a microprocessor (including a DSP) as one of main components of a digital camera and a recent processor has reached a capability of a sufficient throughput as a controller for a camera despite of its low cost. Therefore, a digital camera tends to substitute an image pickup system with digital signal processing for a function of a distance measurement module (AF sensor) component of a silver-salt camera. The technology described above includes mainly xe2x80x9cExploratory methodxe2x80x9d, xe2x80x9cAuto-correlation methodxe2x80x9d, and xe2x80x9cCross-correlation methodxe2x80x9d.
In a focusing system based on the generally known exploratory method, although an AF evaluated value (which is computed from a high-frequency component of the image) at a position where an image is fetched is obtained, it is difficult to detect whether a focal point exists in front of or in back of a position of the focal lens. Thus, AF evaluated values of all over a range where a lens moves are required in order to detect a focus position with a maximum point evaded. Therefore, the whole range is required to be photographed a number of times, and an operation of moving the lens back to the position of a focal point is added, so that it takes a longer time to obtain a correct focal position. In addition, there is another problem such as malfunction when the surrounding is too bright.
In a focusing system based on the auto-correlation method, two or more of different light fluxes are picked up in one frame. In this case, there is a merit that, focus information can be obtained from the fetched image by one frame. On the other hand, there is a drawback that an extremely high peak value is shown at an original position (focal point) of an auto-correlation coefficient because a correlation is obtained based on the identical data. Hence, there is a problem here that it is necessary to depend upon some other system when the position is adjacent to the focal point.
In a focusing system based on the cross-correlation method, two or more of different light fluxes are picked up in different frames respectively, which requires two or more of frames to be prepared. However, there is an advantage that the possibility of occurrence of an extremely high peak value at the original position like in the auto-correlation method is eliminated. Thus, AF information for a range from a position adjacent to the focal point to a distant location can be obtained without being buried in the peak value.
In the above-mentioned cross-correlation method, however, an amount of cross-correlation computations with respect to the whole frames exceeds a practical range so that a long time is required. In addition, it is required to prepare two different light fluxes with a diaphragm plate for AF for obtaining focus information, although such a diaphragm plate for AF is not needed at the time of photography. Furthermore, the aperture of the diaphragm plate for AF is fixed, so that it is disadvantageously difficult to optimize an amount of light received by the image sensor.
In light of the problems described above, it is an object of the present invention to provide an autofocus apparatus enabling a high-speed focusing operation with low cost configuration.
In one aspect of the present invention, there are provided a diaphragm for restricting a light flux from an object as a target for focusing and a diaphragm shift mechanism for shifting the diaphragm in one vertical direction with respect to an optical axis of a focal lens. A portion of an image frame is set as an AF area. In first image-pickup, first image data is acquired for the AF area within the image frame according to a first light flux from the object passing through an aperture section of the diaphragm when the diaphragm is not shifted by the diaphragm shift mechanism. In second image-pickup, second image data is acquired for the AF area within an image frame according to a second light flux from the object passing through the aperture section of the AF when the diaphragm is shifted in one vertical direction with respect to the optical axis by the diaphragm shift mechanism. A cross correlation between the first image data and the second image data is calculated. Then, a distance up to a focus position of the focal lens as well as a direction thereof is computed according to the cross correlation to drive the focal lens to the focus position.
In an another aspect of the present invention, there are provided a diaphragm for restricting a light flux from an object as a target for focusing and a diaphragm shift mechanism for shifting the diaphragm in two vertical directions with respect to an optical axis of a focal lens. A portion of an image frame is set as an AF area. In first image-pickup, first image data is acquired for the AF area within the image frame according to a first light flux from the object passing through an aperture section of the diaphragm when the diaphragm is shifted in one of the vertical directions by the diaphragm shift mechanisms. In second image-pickup, second image data is acquired for the AF area within the image frame according to a second light flux from the object passing through the aperture section of the diaphragm when the diaphragm is shifted in the other one of the vertical directions by the diaphragm shift mechanism. A cross correlation between the first image data and the second image data is computed. Then, a distance up to a focus position of the focal lens as well as a direction thereof is computed according to the cross correlation to drive the focal lens to the focus position.